Animal Exposure and Human Plague, United States, 1970–2017

Since 1970, >50% of patients with plague in the United States had interactions with animals that might have led to infection. Among patients with pneumonic plague, nearly all had animal exposure. Improved understanding of the varied ways in which animal contact might increase risk for infection could enhance prevention messages.

We created a data extraction tool to capture details on patient-animal interactions in the 2 weeks preceding illness onset. If animal exposure had occurred, we classified the type of animal(s) involved as domestic or wild and the interactions as directly or indirectly associated with exposure to Y. pestis. We grouped animal exposures into categories based on authors' judgment regarding risk for transmission. From high-risk to low-risk, the categories were animal bite, scratch, lick, or cough; skinning of a deceased animal; providing care to or handling a sick or deceased animal; co-sleeping; casual handling or touching; and other (walking, feeding, or contact type unspecified). If a patient had >1 animal interaction, the interaction recorded is that of the higher-risk category.
Animal exposure that was plausibly related to plague transmission was identified in 258 (54%) records. The median case-patient age was greater among those with animal exposure (33 years) than those without animal exposure (24 years) (p<0.05). The frequency of known flea bite and mortality rate did not differ between patients with animal exposures and those without animal exposures (Table 1). After peaking in the 1980s, frequency of human plague decreased (Figure). However, the proportion of plague cases with animal exposure seemingly increased over time, from 52% in the years before 2000 to 63% since 2000 (p = 0.07) (Figure).

Conclusions
More than 50% of patients in the United States with plague since 1970 had animal interaction that might have directly or indirectly led to their exposure to Y. pestis. Animals are associated with human plague transmission in varied ways, ranging from direct exposure, such as caring for a plagueinfected animal, to more subtle indirect encounters with infected fleas, such as by co-sleeping with a flea-infested pet in an area with epizootic plague. Nearly all patients with pneumonic plague had animal interaction before illness, and several occurred in an occupational setting. Although the frequency of human plague in the United States has decreased, the proportion of human cases potentially related to animal exposure has concomitantly increased. Cat-associated and wild animal-associated human plague have been documented in previous reports (6)(7)(8)(9). More recently, the severity of plague illness in dogs and the role these animals might play in human plague have been recognized (10). A cluster of pneumonic plague in Colorado was linked to a dog with pneumonic plague (11), and a recent case of canine plague resulted in the potential exposure of >116 persons at a veterinary clinic (12). Gould et al. found that co-sleeping with a dog occurred more frequently among human plague case-patients than among neighborhood controls (13).
Limitations of our analysis include the possibility that human plague cases might have gone undiagnosed and thus were not captured. Our findings might underrepresent animal-associated plague because case records contain variable levels of detail. Thus, some patients might have had animal exposures that were not captured. In many instances, we could not determine which exposure contributed to human illness, if any at all. Therefore, this analysis is meant to describe the potential rather than definitive scope of animal-related human plague.
This report offers perspective on frequency and diversity of animal interaction as possible means of human exposure to Y. pestis in the United States. Given that most human plague worldwide is caused by flea bites, animalassociated prevention messages have been geared toward hunters and trappers, including the use of gloves when handling or skinning wild animals. Our findings highlight One Health-oriented opportunities to maximize plague prevention through communication with veterinarians in plague-endemic areas. Veterinarians play an integral role in plague prevention for animals and humans by increasing use of flea prevention products, promoting basic precautions among pet owners caring for sick pets, and encouraging use of appropriate personal protective equipment in the veterinary community.